Various research efforts have been made with a view to improving corrosion resistance and toughness of a transporting pipe for transporting a fluid containing a corrosive gas such as hydrogen sulfide gas or carbon dioxide gas, and since recently, a clad steel pipe comprising a cladding sheet of high corrosion resistant steel as the inner sheet and a substrate sheet of low-alloy high-strength steel as the outer sheet has been used as the transporting pipe at some localities for testing purposes.
The above-mentioned clad steel pipe is usually manufactured by overlaying a cladding sheet of high corrosion resistant steel with a substrate sheet of low-alloy high-strength steel and pressure-bonding them with each other through hot-rolling to prepare a clad steel sheet; forming said clad steel sheet thus prepared into a blank pipe having said cladding sheet inside and said substrate sheet outside; and welding a seam line of said blank pipe thus obtained.
However, as the service conditions of the clad steel pipe as the transporting pipe have become severer, corrosion resistance of the cladding sheet used in the conventional clad steel pipe has become insufficient. An insufficient corrosion resistance of the cladding sheet results from precipitation of carbides at grain boundaries of the clad steel during preparing the clad steel sheet through hot-rolling.
This problem can be solved by subjecting the clad steel pipe to a solution treatment, through which the clad steel pipe is heated to a prescribed temperature to dissolve the carbides precipitated at the grain boundaries into crystal grains of the cladding sheet, and then, is cooled at a cooling rate that prevents the dissolved carbides from reprecipitating at the grain boundaries.
While the solution treatment of the clad steel pipe improves corrosion resistance of the cladding sheet, the substrate sheet of the clad steel pipe is also affected by the heat treatment similarly to the cladding sheet. The structure of the substrate sheet is thus converted into a hardened structure, thus causing decrease in toughness of the substrate sheet. A clad steel pipe with a decreased toughness of the substrate sheet thereof is not serviceable.
If the clad steel pipe is subjected to a solution treatment and then to a tempering treatment to improve toughness of the substrate steel sheet in an attempt to solve the above-mentioned inconvenience, the cladding sheet is exposed to the same heat treatment as the substrate sheet, thus causing precipitation of carbides at grain boundaries, and hence decrease in corrosion resistance of the cladding sheet.
Because of these problems, it is the present situation that a solution treatment cannot be applied to a clad steel pipe for the purpose of improving corrosion resistance of a cladding sheet.
There is therefore an increasing demand for developing a clad steel pipe having a cladding sheet of high corrosion resistant steel and a substrate sheet of high low-temperature toughness steel. However, such a clad steel pipe has not as yet developed.